1. Field of the Invention
The present invention relates to optical devices, and in particular, to an optical detector which minimizes optical crosstalk.
2. Description of the Related Art
Future high-performance computing will rely on high-density, high-speed electronics linked by high density optical interconnects. Two-dimensional arrays of vertical cavity surface-emitting lasers (VCSELs) will accelerate this evolution. In addition to their use as high density optical interconnects, VCSELs find application in areas ranging from optical communication, optical recording and readout systems, to laser printers and scanners. In the field of optical communications, VCSELs can be advantageously used to provide high bandwidth optical interconnections over fiber optic cable for data communications applications. Because VCSELs can be produced in larger arrays; can be tested in parallel; and because their laser emissions are relatively easy to couple to multi-mode fiber because they emit light perpendicular to the surface of the die; VCSELs have been recognized as an efficient, low cost, alternative to edge emitting semiconductor lasers. It is projected that VCSELs will ultimately dominate the data-networking market, replacing copper as the preferred means of communications for local-area-networking as data-rates reach and exceed 1 Gbit/s.
VCSEL sources are typically driven electrically from separate (off-chip) drivers which significantly increase the power dissipation of the driver-VCSEL package and affect the bandwidth of the link. There is significant opportunity to further increase their efficiency and performance, especially at 10 Gigabit/s and higher rates, by flip-chip bonding VCSEL devices directly to the driver electronics. It is also contemplated that by also flip-chip bonding arrays of optical detectors to a silicon very large scale integration (VLSI) chip, the combined arrays of VCSELs and detectors would comprise a large number of high-speed optical inputs and outputs connected directly to the processing and switching CMOS circuits on-chip.
Conservative assumptions suggest that hybrid I/O technology as described has substantial room for continued scaling to a large numbers of higher-speed interconnects. Future opto-electronic VLSI technologies will provide an I/O bandwidth to a chip that is commensurate with the processing power of the chip, even in the finest linewidth silicon, a task with which conventional electrical interconnect technologies cannot compete. It is anticipated that the availability of hybrid I/O technology will provide a terabit-per-second throughput switch with low power requirements.
In recent years, there has been an ongoing effort to perform flip-chip bonding of photonic devices to silicon CMOS circuits. It has been shown that it is possible to create three-dimensional OE-VLSI structures, with the flip-chip bonded photonic devices placed directly above the silicon CMOS circuitry. See, e.g., K. W. Goossen and A. V. Krishnamoorthy, "Optoelectronics-in-VLSI" Wiley Encyclopedia of Electrical and Electronic Engineering, vol. 15, pp. 380-395, 1999. The photonic devices include optical detectors and modulator devices, and more recently flip-chip bonded VCSEL emitter arrays to CMOS circuits. See A. V. Krishnamoorthy, L. M. F. Chirovsky, et al., "Vertical Cavity Surface Emitting Lasers Flip-chip Bonded to Gigabit/s CMOS Circuits," IEEE Photonics Technology Letters, vol. 11, no. 1, pp. 128-130, January 1999. It has also been demonstrated that it is possible to make multiple sequential attachments of photonic devices to the CMOS circuits (e.g., detectors and lasers). It is therefore shown that the technology of marrying photonic devices to silicon CMOS circuits is a mature technology in the sense that it has been readily demonstrated that both detector and emitter devices can be bonded to silicon VLSI chips. It has also been demonstrated that VCSELs and resonant photodetectors can be fabricated simultaneously on the same wafer and share the same epitaxial growth structure. Although the processing of the photonic devices can become more involved in the latter case, only a single attachment step is required to attach both emitters and detectors to the Silicon CMOS substrate.
One of the key issues in operating large arrays of VCSEL and optical detector devices is to minimize the detrimental effects of crosstalk between the photonic devices and their associated communication channels. Inter-channel crosstalk can be caused by electrical interference between signals on the chip, which can be reduced by proper chip layout and reduction of off-chip parasitics, and can also be due to stray light from another channel unintentionally reaching the incorrect detector. This latter type of optical crosstalk (i.e., stray light) can be quite substantial if the detectors and VCSELs are not designed to prevent this. This is particularly relevant when arrays of VCSEL emitters and optical detectors are interleaved and stray light propagates parallel to the chip. FIG. 1 illustrates the stray light phenomenon for a single VCSEL under operating conditions. Specifically, FIG. 1 is a microphotograph of a VCSEL 10 under lasing conditions as seen with an infrared camera and attenuated by three orders of magnitude. Even when attenuated, one finds that a significant amount of light escapes (i.e., strays) from the surrounding laser mesa. These emissions correspond to spontaneous, and amplified spontaneous emission and scatter from the mesa sidewalls.
FIG. 2 illustrates a conventional VCSEL 21 and optical detector 23 which are grown on the same wafer. It is important to note that there is a high potential for crosstalk due to spontaneous emissions from the active region 25 of the VCSEL 21 and the corresponding active region 27 of the detector 23.
Therefore, there is a need for an optical detector suitable for performing optical signal processing functions in an optical system which minimizes or provides enhanced immunity to crosstalk without necessitating expensive fabrication techniques.